1. Field of the Invention
The present invention relates to a valve timing control system for an internal combustion engine, equipped with a hydraulic actuation type variable valve timing mechanism.
2. Description of the Related Art
FIG. 14 schematically shows a construction of a conventional valve timing control system for use in an internal combustion engine.
As FIG. 14 shows, the conventional valve timing control system is made up of an oil pump 1 for supplying a lubricating oil under pressure to a valve timing control system, a cam shaft 2 for opening and closing intake valves and exhaust valves in the internal combustion engine, a VVT (Variable Valve Timing) actuator 3 for varying the rotational phase of the cam shaft 2 with respect to a crank shaft (not shown), an oil control valve 4 for adjusting the quantity of the lubricating oil to be supplied to the VVT actuator 3, a crank shaft rotational-phase detecting means 5 for detecting a rotational phase of the crank shaft, a cam shaft rotational-phase detecting means 6 for detecting a rotational phase of the cam shaft 2, an operational-state detecting means 7 for detecting an operational state of the internal combustion engine, an oil control valve drive circuit 8 for driving the oil control valve 4 electrically, and an ECU 9 for issuing a command to the oil control valve drive circuit 8.
In this construction, the VVT actuator 3 is composed of a cam pulley 2a rotating in synchronism with the crank shaft (the speed of rotation is 1/2 of that of the crankshaft), and an advance (spark-advance) chamber 3a, a retardation (spark-retardation) chamber 3b and a rotor 3c for rotationally shifting the cam shaft 2 to the advancing side or the retarding side.
Secondly, a description will be given hereinbelow of an operation of the conventional valve timing control system for use in the internal combustion engine.
On the basis of the operational state (engine speed, throttle opening degree, charging efficiency, cooling water temperature and others) of the internal combustion engine the operational-state detecting means 7 detects, the ECU 9 is made to determined the optimal valve timing. Additionally, as a function of the phases of the crank shaft and the cam shaft 2 the crank shaft rotational-phase detecting means 5 and the cam shaft rotational-phase detecting means 6 detect respectively, the ECU 9 calculates the present valve timing.
The ECU 9 calculates a controlled variable (actuation) for the oil control valve 4 so that the deviation between the optimal valve timing and the present valve timing decreases, with this controlled variable being forwarded to the oil control valve drive circuit 8.
The oil control valve drive circuit 8 adjusts the voltage or the current to be supplied to the oil control valve 4 on the basis of a command from the ECU 9 so that the controlled variable commanded by the ECU 9 corresponds to an electrical behavior of the oil control valve 4.
The lubricating oil fed under pressure from the oil pump 1 to the oil control valve 4 is distributed to an advance side oil passage 3d or a retardation side oil passage 3e, communicating with the advance chamber 3a or the retardation chamber 3b, in the oil control valve 4, thereby entering the advance chamber 3a or the retardation chamber 3b.
For the advance of the valve timing, the lubricating oil is supplied into the advance chamber 3a while the lubricating oil in the interior of the retardation chamber 3b is returned through a drain of the oil control valve 4 to an oil pan. Since the cam shaft 2 is connected coaxially with the rotor 3c in the VVT actuator 3, the rotor 3c is rotated by the oil pressure to the advance side with respect to the cam pulley 2a so that the rotational phase of the cam shaft 2 advances relative to the rotational phase of the crank shaft.
On the other hand, for the retardation of the valve timing, the lubricating oil is given to the retardation chamber 3b while the lubricating oil in the interior of the advance chamber 3a is returned through the drain of the oil control valve 4 to the oil pan. This operation reverse to the advancing operation causes the retardation of the valve timing.
The driving force of the VVT actuator 3 is determined from an operational torque of the cam shaft and a pressure of the lubricating oil supplied from the oil pump 1, while the conventional valve timing control system has been designed such that the pressure of the lubricating oil and the operational torque of the cam shaft 2 have been estimated from a detection signal (for example, engine speed or cooling water temperature) of the operational-state detecting means 7.
However, the pressure of the lubricating oil not only depends on the characteristic of the lubricating oil, the variation of the oil temperature and the degree of the deterioration thereof, but also varies, for example, due to the variation of the delivery pressure of the oil pump 1 resulting from the bias of the lubricating oil in the oil pan or the like at the decrease in the oil quantity, at the acceleration/deceleration or at the high-speed turning or due to the increase in the pressure loss stemming from the accumulation of foreign matters in the oil supply passages, even if the internal combustion engine takes the same operating conditions.
In addition, the operational torque of the cam shaft varies with the engine speed and the viscosity of the lubricating oil.
For this reason, there is a problem which arises with the conventional control method of determining the controlled variable by estimating the driving force of the VVT actuator 3 on the basis of only the operational state, however, in that the controllability of the valve timing becomes variable.
Additionally, as the case in which the pressure of the supply oil by the VVT actuator 3 drops significantly, there may be an extremely high oil temperature plus a low engine speed, or the accumulation of foreign matters in the oil supply passages. In these cases, the driving force of the VVT actuator 3 falls, which creates a problem in control of the valve timing to a desired value.
Still additionally, the combustion state of the internal combustion engine becomes unstable.